Excitonic features

Since in quantum wells electrons and holes can freely move only within the quantum well plane, bound electron-hole states, i.e. excitons, become two-dimensional in nature as well. The exciton binding energy is enhanced four-fold in the ideal two-dimensional case compared to a conventional three-dimensional case. In addition, the exciton oscillator strength is also enhanced. In the optical spectra, this leads to pronounced excitonic features which are usually observed even at room temperature. 

Narukawa et al [13] performed electro-reflectance (ER) studies of GalnN/GaN multiple quantum wells, obtaining a rich spectral structure, which was interpreted in terms of various excitonic features. Again, the main ER features are observed at energies which are distinctly higher than the dominating photoluminescence peak. 

For both excitations (Cls or S2p) a pronounced peak occurs at energies just above the Fermi energy. It coincides with the excitonic feature in the p-XAS curves. 

Notably, pronounced excitonic features inherent in ZnSe single crystals remain in DBR-stmctures. This makes possible to use strong excitonic optical noidinearities which are known for ZnSe single crystal [7] to get nonlinear DBR-stmctures with... 

CP24 and LHCI-680 absorb at about 674 nm. Appreciable differences are also observed in the chi b shoulder. This is at 641 nm in CP29 while at 651 nm in LHCII and CP24. CP26 exhibits both chi b absorption forms. Chi b in LHCI-680 and 730 absorbs with a small peak at 645 nm (5).Circular dichroism spectra are shown in fig. 5. They were obtained from samples dissolved in 0.06% DM. In the LHCII spectrum all the excitonic features previously... 

If the spectrometer has a large acceptance angle, a and ji contributions are mixed and the spectrum represents the total DOS (density of states) of the unoccupied states (including a and jr MOs) [22]. There is some doubt whether the exact absolute position and intensity distribution of the first three peaks (a, b, c in Table 4/5) have to be attributed to core-exciton features or to the local density of states effect [22]. Based on an intensity analysis of boron and... 

Figure 32 Electrically tunable energy transfer from a single semiconductor nanorod to a dye molecule. High-resolution (a) and overview (b) transmission electron micrographs showing the structure of the CdSe/CdS nanocrystals used, (c) For a specific set of a single nanocrystal and a single dye molecule no energy transfer occurs because of the lack of spectral overlap between nanocrystal emission and dye absorption, (d) After application of an electric field, the nanocrystal s PL is red shifted, resulting in the resonance of the nanocrystal and dye transitions. This leads to energy transfer to the dye and subsequent emission, (e) Absorption (dashed lines) and PL (solid lines) spectra of nanocrystals (blue lines) and dye (red lines). Absorption spectra were measured in chloroform solution at room temperature, whereas emission spectra in polystyrene/dye blends at 50 K. Note the considerable spectral overlap of nanocrystal emission with dye absorption. The inset shows the solution absorption and PL of the nanocrystal excitonic feature. (Reprinted by permission from Macmillan Publishers Ltd from Ref. 72.)...

The intensity of the bound-exciton peaks, relative to that of the free-exciton features, gives an indication of the uncompensated boron concentration in the region of the diamond examined. For the diamond shown in Fig. 6b this is about 5 X 10 cm as determined from Hall effect measurements (31). A very weak peak D, due to the accidental presence of a small concentration of boron (estimated as 3 X 10 cm ), is also evident in the low-temperature spectrum in Fig. 5. 

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